Why is it 不请自来 instead of 沒请自来 (沒受邀請, 便自行到來)?
你不願邀請我, 我便厚著臉皮, 自行到來 (不被待見卻自行到來) ==> 不请自来 is usually (although not always) applied to situations where one's visit is not only uninvited (without having been asked) but also unwelcome (not gladly received).
你老这么躲着我 (你不願見我), 我只好不请自来了 (knowing that I'm not welcome).
不请自来的客人(不請他卻自行來的客人)比沒請自來的客人(不速之客)更不受歡迎